Description (taken directly from the application): Allogeneic bone marrow transplantation from unaffected donors to prediabetic hosts can prevent the development of autoimmune disease, albeit with the cost of graft vs host (GvH) disease. We have transplanted purified hematopoietic stem cells (HSCs) form AKR/J mice to lethally or sublethally irradiated prediabetic NOD mice, creating chimeras (with both AKR/J and NOD T cells) and preventing the progression from insulitis to diabetes; no GvH or HvG T cell immunity can be detected. While this treatment has gratifying results and serves as a potential precedent for future clinical trials, the mechanism by which these chimeric mice block autoimmune T cells is unknown. Traversing the AKR/J to NOD barrier bypasses a significant fraction of the more than 14 gene loci believed to play a role in NOD diabetes. Here we begin to assign some of these genes to donor vs host by transferring HSCs across fewer genetic barriers. Because at least part of the NOD diabetogenic process will likely include thymic positive selection and migration of some diabetogenic T cells, and this process is likely to be altered in chimeras, we focus much of our research attention to documenting in NOD mice, in HSC chimeric NOD mice, and in TCR-Tg NOD mice (in collaboration with M. Davis, project 4) the number and TCR specificity of the four distinct stages in thymic development that are engaged in positive and negative selection. We shall use as reagents to detect the progress of diabetogenic thymic subsets either the IA -peptide reagents generated by YH Chien (Project 5) for NOD and chimeric NOD mice, and anti-TCR reagents for TCR-Tg mice. Our TCR- Tg mice will be constructed with endogenous TCR and TCR enhancers in order to allow the ordered increase in TCR levels in a stage-specific manner; TCR transgenes with Ig enhancers do not show this regulation. Functional alterations in NOD mice, in HSC chimeras and in TCR "knock- in" mice will be tested for selection in thymic stromal cultures, and selected thymic and peripheral T subsets will be adoptively transferred to NOD neonates to assess their diabetogenic (or blocking) potential.